[PATCH v2] um: time-travel: fix signal blocking race/hang

Johannes Berg
2024-07-03T11:01:57Z

From: Johannes Berg <johannes.berg@intel.com>

When signals are hard-blocked in order to do time-travel
socket processing, we set signals_blocked and then handle
SIGIO signals by setting the SIGIO bit in signals_pending.
When unblocking, we first set signals_blocked to 0, and
then handle all pending signals. We have to set it first,
so that we can again properly block/unblock inside the
unblock, if the time-travel handlers need to be processed.

Unfortunately, this is racy. We can get into this situation:

// signals_pending = SIGIO_MASK

unblock_signals_hard()
signals_blocked = 0;
if (signals_pending && signals_enabled) {
block_signals();
unblock_signals()
...
sig_handler_common(SIGIO, NULL, NULL);
sigio_handler()
...
sigio_reg_handler()
irq_do_timetravel_handler()
reg->timetravel_handler() ==
vu_req_interrupt_comm_handler()
vu_req_read_message()
vhost_user_recv_req()
vhost_user_recv()
vhost_user_recv_header()
// reads 12 bytes header of
// 20 bytes message
<-- receive SIGIO here <--
sig_handler()
int enabled = signals_enabled; // 1
if ((signals_blocked || !enabled) && (sig == SIGIO)) {
if (!signals_blocked && time_travel_mode == TT_MODE_EXTERNAL)
sigio_run_timetravel_handlers()
_sigio_handler()
sigio_reg_handler()
... as above ...
vhost_user_recv_header()
// reads 8 bytes that were message payload
// as if it were header - but aborts since
// it then gets -EAGAIN
...
--> end signal handler -->
// continue in vhost_user_recv()
// full_read() for 8 bytes payload busy loops
// entire process hangs here

Conceptually, to fix this, we need to ensure that the
signal handler cannot run while we hard-unblock signals.
The thing that makes this more complex is that we can be
doing hard-block/unblock while unblocking. Introduce a
new signals_blocked_pending variable that we can keep at
non-zero as long as pending signals are being processed,
then we only need to ensure it's decremented safely and
the signal handler will only increment it if it's already
non-zero (or signals_blocked is set, of course.)

Note also that only the outermost call to hard-unblock is
allowed to decrement signals_blocked_pending, since it
could otherwise reach zero in an inner call, and leave
the same race happening if the timetravel_handler loops,
but that's basically required of it.

Fixes: d6b399a0e02a ("um: time-travel/signals: fix ndelay() in interrupt")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
---
v2: use __atomic_*() primitives, and do so consistently
---
arch/um/os-Linux/signal.c | 118 +++++++++++++++++++++++++++++++-------
1 file changed, 98 insertions(+), 20 deletions(-)

diff --git a/arch/um/os-Linux/signal.c b/arch/um/os-Linux/signal.c
index 787cfb9a0308..b11ed66c8bb0 100644
--- a/arch/um/os-Linux/signal.c
+++ b/arch/um/os-Linux/signal.c
@@ -8,6 +8,7 @@

#include <stdlib.h>
#include <stdarg.h>
+#include <stdbool.h>
#include <errno.h>
#include <signal.h>
#include <string.h>
@@ -65,9 +66,7 @@ static void sig_handler_common(int sig, struct siginfo *si, mcontext_t *mc)

int signals_enabled;
#ifdef UML_CONFIG_UML_TIME_TRAVEL_SUPPORT
-static int signals_blocked;
-#else
-#define signals_blocked 0
+static int signals_blocked, signals_blocked_pending;
#endif
static unsigned int signals_pending;
static unsigned int signals_active = 0;
@@ -76,14 +75,27 @@ static void sig_handler(int sig, struct siginfo *si, mcontext_t *mc)
{
int enabled = signals_enabled;

- if ((signals_blocked || !enabled) && (sig == SIGIO)) {
+#ifdef UML_CONFIG_UML_TIME_TRAVEL_SUPPORT
+ if ((signals_blocked ||
+ __atomic_load_n(&signals_blocked_pending, __ATOMIC_SEQ_CST)) &&
+ (sig == SIGIO)) {
+ /* increment so unblock will do another round */
+ __atomic_add_fetch(&signals_blocked_pending, 1,
+ __ATOMIC_SEQ_CST);
+ return;
+ }
+#endif
+
+ if (!enabled && (sig == SIGIO)) {
/*
* In TT_MODE_EXTERNAL, need to still call time-travel
- * handlers unless signals are also blocked for the
- * external time message processing. This will mark
- * signals_pending by itself (only if necessary.)
+ * handlers. This will mark signals_pending by itself
+ * (only if necessary.)
+ * Note we won't get here if signals are hard-blocked
+ * (which is handled above), in that case the hard-
+ * unblock will handle things.
*/
- if (!signals_blocked && time_travel_mode == TT_MODE_EXTERNAL)
+ if (time_travel_mode == TT_MODE_EXTERNAL)
sigio_run_timetravel_handlers();
else
signals_pending |= SIGIO_MASK;
@@ -380,33 +392,99 @@ int um_set_signals_trace(int enable)
#ifdef UML_CONFIG_UML_TIME_TRAVEL_SUPPORT
void mark_sigio_pending(void)
{
+ /*
+ * It would seem that this should be atomic so
+ * it isn't a read-modify-write with a signal
+ * that could happen in the middle, losing the
+ * value set by the signal.
+ *
+ * However, this function is only called when in
+ * time-travel=ext simulation mode, in which case
+ * the only signal ever pending is SIGIO, which
+ * is blocked while this can be called, and the
+ * timer signal (SIGALRM) cannot happen.
+ */
signals_pending |= SIGIO_MASK;
}

void block_signals_hard(void)
{
- if (signals_blocked)
- return;
- signals_blocked = 1;
+ signals_blocked++;
barrier();
}

void unblock_signals_hard(void)
{
+ static bool unblocking;
+
if (!signals_blocked)
+ panic("unblocking signals while not blocked");
+
+ if (--signals_blocked)
return;
- /* Must be set to 0 before we check the pending bits etc. */
- signals_blocked = 0;
+ /*
+ * Must be set to 0 before we check pending so the
+ * SIGIO handler will run as normal unless we're still
+ * going to process signals_blocked_pending.
+ */
barrier();

- if (signals_pending && signals_enabled) {
- /* this is a bit inefficient, but that's not really important */
- block_signals();
- unblock_signals();
- } else if (signals_pending & SIGIO_MASK) {
- /* we need to run time-travel handlers even if not enabled */
- sigio_run_timetravel_handlers();
+ /*
+ * Note that block_signals_hard()/unblock_signals_hard() can be called
+ * within the unblock_signals()/sigio_run_timetravel_handlers() below.
+ * This would still be prone to race conditions since it's actually a
+ * call _within_ e.g. vu_req_read_message(), where we observed this
+ * issue, which loops. Thus, if the inner call handles the recorded
+ * pending signals, we can get out of the inner call with the real
+ * signal hander no longer blocked, and still have a race. Thus don't
+ * handle unblocking in the inner call, if it happens, but only in
+ * the outermost call - 'unblocking' serves as an ownership for the
+ * signals_blocked_pending decrement.
+ */
+ if (unblocking)
+ return;
+ unblocking = true;
+
+ while (__atomic_load_n(&signals_blocked_pending, __ATOMIC_SEQ_CST)) {
+ if (signals_enabled) {
+ /* signals are enabled so we can touch this */
+ signals_pending |= SIGIO_MASK;
+ /*
+ * this is a bit inefficient, but that's
+ * not really important
+ */
+ block_signals();
+ unblock_signals();
+ } else {
+ /*
+ * we need to run time-travel handlers even
+ * if not enabled
+ */
+ sigio_run_timetravel_handlers();
+ }
+
+ /*
+ * The decrement of signals_blocked_pending must be atomic so
+ * that the signal handler will either happen before or after
+ * the decrement, not during a read-modify-write:
+ * - If it happens before, it can increment it and we'll
+ * decrement it and do another round in the loop.
+ * - If it happens after it'll see 0 for both signals_blocked
+ * and signals_blocked_pending and thus run the handler as
+ * usual (subject to signals_enabled, but that's unrelated.)
+ *
+ * Note that a call to unblock_signals_hard() within the calls
+ * to unblock_signals() or sigio_run_timetravel_handlers() above
+ * will do nothing due to the 'unblocking' state, so this cannot
+ * underflow as the only one decrementing will be the outermost
+ * one.
+ */
+ if (__atomic_sub_fetch(&signals_blocked_pending, 1,
+ __ATOMIC_SEQ_CST) < 0)
+ panic("signals_blocked_pending underflow");
}
+
+ unblocking = false;
}
#endif

--
2.45.2